So I modified it as shown in the following diagram which
more closely simulates a 74C/HC14Schmitt
trigger including inversion, reasonably high input
resistance (4M) and the all important input protection
diodes.

Then I decided to write a little description of operation
and before you know it turned into a tutorial. The Schmitt
trigger is a nice example of positive feedback so trying
to describe that concept took another few paragraphs.

"Why?" you may ask. Fourteen components may seem like a
lot to simulate 1/6 of one 74HC14,
but this discrete circuit shows you what is really going on
inside that little black IC we so often take for
granted.

The Schmitt
trigger is used in BEAM for
Nv
and Nuneurons
but is more generally used to clean up analog voltages and
convert them into nice sanitary binary logic levels. If you
are in a hurry you can skip straight ahead to the section on
the Schmitt
trigger but if you like some background read the bit
about digital and analog signals and feedback first.

DIGITAL SIGNALSDigital signals are generally considered to be one of
two logic states called by various names: On / Off, High /
Low, One / Zero, 1 / 0 and Vcc / Gnd. These are called logic
signals because they are unambiguous "either / or"
values.

In the real world, the voltage levels that represent
logic states are not precisely Vcc and Gnd. When these fuzzy
real voltage levels are applied to digital inverter inputs,
they are compared to an internal voltage threshold and the
difference between the applied input voltage and the
threshold is amplified (multiplied) by a factor of about
100. These real voltage levels representing 1 and 0 must be
in a range of values, sufficiently above and below the
actual switching threshold so that despite their fuzzyness
(noise) they always generate logic 1 or 0 levels at the
output of the circuit.

In 74HCxx logic logic "1" signal levels are defined as
greater than 2/3 Vcc and a logic "0" signal must be less
than 1/3 Vcc. The range of voltage levels between 1/3 Vcc
and 2/3 Vcc is considered to be in the "forbidden" zone
since signals in that range cannot be guaranteed to be 1 or
0. When a voltage near the switching threshold in the middle
of the "forbidden" zone is applied to a digital input, the
digital output can and does generate a burst of pulses which
can play havoc in the precise digital world of counters and
registers.

In the real world, we also have to consider that when a 1
changes to a 0 and vice versa, the change is not
instantaneous and while the logic voltage level slews from 1
to 0, it traverses through the "forbidden" zone. But since
digital logic cannot react instantaneously to change either,
there is a specification given for the minimum rate of
change (switching time) which is guaranteed not to generate
more than a single transition in response to the logic level
change.

So our unambiguous digital logic ignores glitches as long
as the time of change is sufficiently short duration (<
500 ns). In short, a digital circuit responds to and
generates one of two digital voltage levels, nominally
labeled "1" and "0". These levels must be above and below
the "forbidden" zone and any changes in logic levels must be
sufficiently rapid to meet the minimum specified switching
times. Perhaps now is a good time to mention that in the
world of BEAM
circuits, these rules are generally ignored. Also note that
On / One / 1 / High is generally, but not always, equivalent
to Vcc; meanwhile, Off / Zero / 0 / Low is generally, but
not always, equivalent to Gnd. Sometimes this common-sense
correlation is reversed (so that "On" is denoted by Gnd, and
"Off" by Vcc) -- this is called "negative
logic," and happens to be used in microcores.

But I digress.

ANALOG SIGNALSDigital signals are to analog signals what black and
white is to the full spectrum of colors. Analog signal
levels can be any value and each value is significant. In
the real world there is a limit to the minimum and maximum
values that can be distinguished by the input of an analog
circuit and the values that can be generated at the output
of an analog circuit. Usually these signals fall in the
range between the circuit power supply voltages. The main
thing to remember is that sensors such as such as LDRs,
PDs,
thermistors and time sensitive circuits, such as RC networks
(Nv
/ Nu),
all generate analog voltages including those that are in the
digital "forbidden" zone.

In order to interface the analog and digital worlds we
must use special circuit designs to avoid generating
unpredictable chaotic results. This is done by using
positive feedback.

FEEDBACKOne of the most important concepts in electronics is
feedback. Once you understand this basic principle
which applies to all dynamical systems, you will experience
a quantum jump in knowledge. If I may be so bold to suggest
this idea: Chaos, Order and Feedback rule in a delicate
balance that gives rise to all phenomena in this
universe.

Feedback, as the name implies, occurs when a process or
interaction is recursively modified by the output or result
that it generates. In electronic circuits this occurs when
all or a part of the output signal(s) is added or subtracted
from the input signal(s). Feedback therefore has two
distinct forms: Positive and Negative. To keep things simple
and on familiar ground, we will just discuss in general how
BEAM
circuits use feedback and in detail how positive feedback is
used in Schmitt
trigger circuits.

ELECTROMECHANICAL FEEDBACKThe essence of BEAM is the "autonomous" interaction
between the electronic and mechanical assembly called the
robot and it's environment as seen through it's sensors. The
sensors provide input signals which modify the action of the
robot and, in turn, that action modifies the signals
received by the sensors. Well that is a fine example of
electromechanical feedback!

In a phototropic robot like a photopopper, the circuit
that controls the motors sends more current pulses to the
side that receives less light. This causes that side to turn
towards the light source until both light sensors are
balanced. Then both sides receive equal current pulses as
the robot "waggles" towards the light.

In the Herbie line follower, the motion is continuous
rather than pulsed and the robot follows a broad white line
against a dark background. Each motor receives current in
proportion to the imbalance (also called error or difference
signal) of light on the two photo sensors that point to the
left and right edges of the white line. For example, as the
'bot drifts left, off the center of the line, the left
sensor receives less light as it moves over to the dark
background while the right sensor receives more from the
center of the reflective white line therefore creating an
imbalance signal that increases the current to the right
motor and moves Herbie back on track.

Phototropism is an example of negative feedback
because the system as a whole moves towards the "balanced
sensors signals" condition. This is a very important
distinction from the "maximum sensors signals" condition.
Photophobic behavior is and example of positive
feedback that steers the system as a whole towards
"unbalanced sensor signals" rather than the "minimum sensor
signals" condition. However don't let these subtle
distinctions get in the way of the main idea that the action
of the system influences the sensors which influences action
of the system and so on - that is the feedback loop we so
often mention in the discussion of BEAM
circuits.

ELECTRONIC FEEDBACK
If you got the idea of mechanical feedback, then electronic
feedback should be easy. In BEAM
type applications which use digital inverters for quasi
analog applications , the feedback balance point is the
input voltage level (threshold) at which the output switches
over. For 74HCxx inverters like the 74HC240
that level is 1/2 Vcc, right smack in the middle of the
"forbidden" zone. We mentioned earlier that negative
feedback subtracts from the input signal and steers the
circuit output towards the balance point. Positive feedback
adds to the input signal and steers the circuit output away
from the balance point.

NEGATIVE FEEDBACK
Negative feedback subtracts from the input signal because it
is inverted before it appears at the output and any inverted
output signal will subtract from the input signal.

A good example of negative feedback would be a 74HC240
inverter with a resistor connected from input to output. If
you measure the output of that circuit with a voltmeter you
will know precisely what the threshold voltage of the
inverter is. For a 74HC240
at Vcc = 5 V the output will be very close to 2.5 V. As
mentioned before digital logic is not designed to operate
with input voltages from the forbidden zone and if you
measure the Vcc current you will know why it draws 50 mA or
more current. In addition, if you turn on a radio near the
circuit, the high frequency oscillation radiated from the
circuit should be quite overpowering compared to local radio
stations.

In general, negative feedback is undesirable in digital
circuits but it can be harnessed and put to good use as will
be discussed later.

POSITIVE FEEDBACKPositive feedback adds to the input signal and steers
the output away from the balance point, out of the forbidden
zone and towards the Vcc or Gnd levels of ideal logic
signals. This is why positive feedback is generally useful
in BEAM circuits and can in fact be
used to counteract the effects of negative feedback.

There are many examples of external positive feedback in
BEAM bicores, monocores, SE triggers
and latches. But the 74HC14
of Microcore
fame, is an example of internal positive feedback because
the feedback occurs inside the chip. As a black box, all we
know about the 74HC14
is that it has two thresholds and that regardless of the
input signal, the output signals always have nice clean
single transitions, are always at Vcc or Gnd and never at
some in between value (unless we forget to add a resistor in
series with the LED
indicators). This internal positive feedback is what makes
the 74HC14Microcore
work and why, without positive feedback, a 74HC240Microcore
always degenerates into saturation.

SCHMITT TRIGGERSThe Schmitt
trigger is a special circuit which acts like a switch
that changes state at two different thresholds. These are
called the upper and lower threshold or the positive and
negative going threshold. The difference in these two
threshold levels is called the hysteresis
voltage.The Schmitt
trigger does not react to any input voltage level in the
range between the two thresholds, which for a 74C/HC14
corresponds precisely to the "forbidden zone".

A Schmitt
trigger can also be likened to two comparators
controlling an RS flip-flop at the output. In fact, the
schematic of the 74C14
shows it to be designed that way. The upper threshold
comparator sets the output latch and the lower threshold
comparator resets the output latch.

These two thresholds (balance points) makes the 74HC14Schmitt
trigger different from an ordinary 74HC240
inverter with a single threshold at 1/2 Vcc.

Normally the 74HC14
threshold parameters are a fixed ratio of Vcc. This keeps
the device functionally simple and as a result the 74HC14Schmitt
trigger is one of the most popular devices for
interfacing real world signals to digital electronics. It
just doesn't get any simpler compared to the other versions
we will discuss.

However it is a little known fact that it is possible to
alter the thresholds of a 74HC14
by using negative feedback from output to input. For
example, by adding a 5.1M resistor from output to input and
a 1M input resistor this will provide about 15% negative
feedback. That is subtracted from the internal 30% positive
feedback and with Vcc=5V, it effectively changes the
thresholds at the input of the 1M resistor to approximately
2.1V and 2.9V. Very useful if the signal of interest has
smaller transistions than the normal 74HC14
hysteresis voltage.

Beside the 74HC14,
there are a number of ways to impliment a Schmitt
trigger in CMOS logic. The simplest is to use a
non-inverting buffer like the 74HC245
and connect a 3M resistor from output to input to provide
positive feedback which is summed with the input signal
through the 1M series resistor. These values will give the
same thresholds as a 74HC14
but keep in mind the input resistance is 4M to GND or Vccc
depending on the current output state.

The same circuit can be achived with two inverters like
the 74HC240
version shown. The two inversions of the signal generate the
required positive feedback. Both the true and inverted
output signals are available. One variation on the last
circuit is to add some negative feedback with a 4.7M
resistor from the inverted output to the input. This cancels
out part of the positve feedback and reduces the hysteresis
voltage while permitting a larger input resistor.

Ideal amplifiers called opamps (a.k.a., operational
amplifiers) can be used to make a simple (compared with the
discrete version) Schmitt
trigger but don't let it lure you away from the object
of the exercise showing just what goes on "under the hood"
of a transistor Schmitt
trigger:

I have included the inverting and non-inverting examples
of opamp Schmitt
trigger which have adjustable threshold and hysteresis
voltage. Note that in both cases the potentiometers settings
interact so that the threshold and hysteresis must be
adjusted by trial and error.

The above diagram reproduces the basic Schmitt
trigger circuit of Richard Piotter. It consist of two
inverters (NPN
and PNP)
which give a double inversion to the input signal. The
output of the second stage is fed back and summed with the
input signal and a resistor
to ground.
Think of those resistors
as forming a voltage divider which determines the input
voltage required to cross the 0.6 V threshold of the
NPNbaseemitter
junction to turn the transistor
on of off. With the values given the positive going
threshold is 1.95 V and the negative going threshold is 1.34
V assuming a Vcc of 5V. The output signal at the PNP
collector is non-inverting with respect to the input
signal.

This diagram shows how the basic circuit is modified to
give the symmetrical 1/3 - 2/3 Vcc thresholds equal to the
74C/HC14Schmitt
trigger. This is done by setting NPNemitter
voltage to 1/2 Vcc - 0.6 V which makes the on / off
switching threshold at the input NPN
base exactly 1/2 Vcc. The 1M input and 3M feedback resistors
form a voltage divider that sets the values of the positive
going input threshold to 2/3 Vcc and the negative going
input threshold is 1/3 Vcc.

The positive feedback signal at one end of the 3M
feedback resistor
is alternately Vcc or Gnd depending on the state of the
non-inverted output signal at the collector of the PNPtransistor.
The other end of the 3M feedback resistor
is at the base of the input NPN
which is at 1/2 Vcc during switching. The voltage at the
input of the 1M resistor
is therefore 1/3 * 1/2 Vcc = 1/6 Vcc above and below 1/2 Vcc
which at Vcc = 5V is 3.33 V and 1.66 V respectively. This
ignores the effect of the <0.1 uA that sinks into the
base of the NPN
at switching.

The inverting NPN
output stage provides isolation, and input protection diodes
were added to simulate the 74C/HC14
inverting Schmitt
trigger so that this circuit can now be used in the same
kind of applications as that device but at Vcc up to 24 V or
higher depending on the transistors. The NPN
output drive to positive is limited by the 4.7K resistor
but it could be replaced with a smaller resistor
or even a pager motor. The resistor
in the PNP
collector was chosen for low power but can be replaced with
4.7K to increase base drive for the NPN
output transistor.

The input diodes are not needed for protection since the
1M resistor
takes care of that but may be needed to clamp an input
coupling capacitor
(i.e., Nv)
to the negative and positive rails. This is called DC
restoration since it removes the residual charge from the
capacitor
so it has no memory of any previous switching operation
which might otherwise affect the timing of a subsequent
switching operation.

While this circuit is not an economical substitute for
the 74HC14,
it gives a good insight into the design of trigger circuits
in general.

Enjoy

wilf

For more information...

The material in this page is an edited and updated
(based on email conversations) version of a
BEAM-list
posting by Wilf Rigter.